This invention relates to an element analyzing apparatus for analyzing elements of a sample by mass spectrometry of secondary ions emitted from the sample which is irradiated by ions to excite compositions of the sample.
FIG. 1 schematically illustrates a hitherto used element analyzing apparatus. This apparatus includes an ion gun 1 used as a secondary ion excitation source, an energy filter 2 used as an ion optical system, and a quadrupole mass spectrometer 3. Reference numeral 4 denotes a sample from which trajectories of the secondary ions 7 start. The energy filter 2 consists of an entry aperture plate 21, an exit aperture plate 22 (the numerals 21 and 22 denote the apertures themselves hereinafter), a deflector plate 23 and an energy filter control power source 24. The quadrupole mass spectrometer 3 consists of a quadrupole 31, a quadrupole control power source 32 and a detector 33. The energy filter 2 serves to collect and transport the secondary ions and to remove high-energy ions forming an obstructive background for energy spectra.
FIG. 2 illustrates a typical energy distribution of secondary ions. This holds true in almost all of the elements, for example, B (boron). The energy distribution starting from 0 eV increases to the maximum value of the ion number in the proximity of 10 eV and then progressively decreases to the value near to 10 eV. In this graph, an energy value at a point where its ion number is 1/e times the maximum ion number is referred to herein as "secondary ion energy constant", where e is the base of natural logarithm. Such an energy value Eos is about 20 eV.
This value often varies to 10 eV or 30 eV depending upon combined conditions of the elements. However, the shape of the distribution itself does not change greatly. The secondary ion energy constant Eos is defined as a standard for indicating a degree of extension of the secondary ion energy distribution shown by a decreasing curve similar to an exponential function. The ion energy constant Eos is essential for quantitatively analyzing the mass spectrometer analysis using the secondary ions, as is clear from the explanation hereinafter. Such a standard has been only vaguely considered in the prior art and has not been strictly defined. As can be seen from FIG. 2, the secondary ion included from the energy values 0 eV to the secondary ion energy constant Eos are the majority of the secondary ions emitted from the sample. Moreover, almost all of the secondary ions are included in the range to Eos, that is, twice the secondary ion energy constant.
Moreover, it has been known that the secondary ions 7 emitted from the sample 4 traject in all directions.
In the prior art, the secondary ions hardly accelerate between the sample 4 and the energy filter 2. Therefore, the ion transmission energy between the energy filter 2 and the quadrupole mass spectrometer 3 is substantially equal to the emission energy of the secondary ions. The energy filter 2 of the prior art apparatus exhibits a resolution of approximately 0.2 or less owing to its structural limitation. In other words, the energy filter 2 has an energy transmission range of 0.2 or less times the transmission energy.
In general, moreover, the maximum possible analyzing energy value of a quadrupole mass spectrometer is desired to be as low as possible in consideration of technical difficulty and economy. In the examples of the prior art, therefore, the maximum possible analyzing energy value of a quadrupole mass spectrometer is usually set of 2Eos, that is, twice the secondary ion energy constant or approximately 40 eV or less in commensuration with the secondary ion energy distribution.
The apparatus described above has the following problems.
In the prior art, only a small portion of the secondary ions emitted accidentally toward the entry aperture 21 of the energy filter 2 is collected in the energy filter 2 and detected among the secondary ions 7 emitted from the sample 4 in all the directions as above described. Accordingly, the collecting efficiency of the secondary ions 7 with emitting angular distribution is very low, usually 0.01-0.1.
In the prior art, moreover, only the small portion of the secondary ions having energy values within the transmission energy range of the energy filter 2 is collected in the energy filter 2 and detected among the secondary ions 7 having a wide energy distribution. Namely, the collection efficiency of the secondary ions with the emitted energy distribution is also very low, usually 0.01-0.1.
The sensitivity of the elementary analysis of the prior art apparatus is very due to these problems.
Several examples which are probably related to the invention of the present application are hereby described. There have been reports, lacking in clear documentary descriptions, showing electric potentials of samples being raised to values in the order of 10V for respective particular purposes. In these examples, the sensitivity is further lowered in comparison with the case where the electric potentials are 0 eV. This results probably from the fact that as the secondary ions are accelerated only by values of in the order of 10 eV, the transmission energy in the quadrupole mass spectrometer increases, so as to decrease the energy of the secondary ions which have energy less than the above described maximum analyzing possible energy and are capable of being normally subjected to the mass spectrometry.
In one example of an experiment (Japanese Society for the Promotion of Science, No. 141 committee, material P. 6-P. 11 for No. 43 research meeting), it was reported that when potential of a sample is a few volts, the highest sensitivity obtained is about ten times higher than when the potential of the sample is at zero volt. In this case, however, a gentle potential gradient in close proximity to a surface of a sample has the particular effect of causing the amount of emitted secondary ions to increase. In this example, when the potential exceeds the value corresponding to the highest sensitivity, such a particular effect rapidly disappears so as to lower the sensitivity in conjunction with the reason above described.
The assignee of the present invention proposed in Japanese Patent Application No. 93,609/84 filed on May 10, 1984 that an imaging type energy filter having image observation function between a sample 4 and a quadrupole mass spectrometer be provided, thereby eliminating crater effect and enabling local analysis to be effected. In case that such an imaging type energy filter is located, it is first necessary to delete the influence of chromatic observation for the prior art apparatus, so that energy resolution needs to be improved twice or to the order of 0.1. Therefore, this apparatus encounters a new problem of selecting whether the collection efficiency for the energy distribution is lowered further or space resolution is spoiled.
In this case, moreover, the secondary ions are not accelerated and pass under low energy condition through the ion optical system. Accordingly, a spherical aberration coefficient becomes very large. Such a large spherical aberration coefficient gives rise to a new limitation of space resolution.
With the quadrupole mass spectrometer, moreover, there is inherently an essential disadvantage (fringing effect) in that all ions passing therethrough are not all detected due to potential disturbance at an end of the quadrupole. As is well known, this detrimental effect of the potential disturbance becomes larger as the transmission energy becomes smaller. Therefore, this is a very serious problem for the apparatus of the prior art.
In general, furthermore, there is another problem in that when the transmission energy is small, the ions are susceptible to charge-up resulting from space charge and electrode contamination to lower the efficiency and increase the instability.
In order to solve these problems, there are some apparatuses of the prior art wherein the potential of an energy filter is lowered only at the trajectories of the ions for increasing the transmission energy in the energy filter so as to decelerate the ions immediately before a quadrupole mass spectrometer, thereby restoring the transmission energy in a quadrupole. (This is the case when positive ions are detected. In case of negative ions, the potential is raised. The explanation will concern the positive ions, hereinafter.) Such an apparatus has the possibility of solving part of the above problems. However, the deceleration immediately before the quadrupole causes scattering of the ions, so that quantity of the secondary ions incident onto the quadrupole mass spectrometer and normally subjected to the mass spectrometry is substantially equal to that in the case of no acceleration. Therefore, it does not solve these problems fundamentally. As can be seen from the theorem of charged beam emittance invariability, this is a natural result.
The theorem of emittance invariability will be explained in somewhat greater detail hereinafter, because it is in connection with the effect of the present invention.
The charged beams are inherently of an aggregation of various trajectories. In consideration of a certain sectional surface (which is assumed as axial symmetry), respective trajectories have different positions (which means distances from the axis) and different energies in transverse directions. An area of a closed curve consisting of a collection of points in a phase diagram having two axes of these two quantities is usually referred to as "emittance". A quantitative value of the emittance determines a quality of beams. The smaller the emittance, the better in quality are the beams with short distances from the axis and less energy in transverse directions. According to the theorem of emittance invariability, so long as the energies in axial directions are equal, no matter how beam conditions may be changed, the emittance itself will remain static. This is an important characteristic of the emittance. Namely, even if acceleration and deceleration are effected on the way, and even if diffusion and convergence of beams occur by lenses, so long as the ultimate axial energy is equal to the initial energy, the emittance stays constant even if a shape of a closed curve in a phase diagram is changed. In order to make the emittance small, there is no way other than making the axial energy large by acceleration.
The secondary ions emitted from a sample have different positions corresponding to the analysis zone and different transverse energies corresponding to emitting angle distribution and inherently have a substantially large emittance.
Then, no matter how the secondary ions may be controlled by the use of lenses and the like, so long as the ultimate axial energies or the transmission energies are equal, when diameters or positions of beams are made small, the transverse energies become large. And when the transverse energies are made small, the diameters of the beams become large, so that the emittance stays constant. On the other hand, the quadrupole mass spectrometer does the mass spectrometry normally only for secondary ions among the incident ions which are within ranges as to diameters or position and transverse energies. In order to enlarge the range, it is necessary to increase the impressed voltage and frequency. However, such an increase is so difficult technically that with standard apparatuses where only narrow ranges are employed, only diameters of the order of 4 mm and transverse energies of the order of about 4 eV are possible.
In the apparatus of the prior art described above the emitted secondary ions are accelerated and pass through the energy filter under an energy condition higher than that when emitted. As a result, the emittance becomes small, so that only the energy filter exhibits a high collection efficiency. However, if the secondary ions are decelerated immediately before the quadrupole to original energy values, the emittance returns to its inherent value, so that the number of the secondary ions within ranges to be normally analyzed by the quadrupole mass spectrometer becomes very small, which is ultimately equal to the case of no acceleration. An improvement of the collection efficiency as to the energy distribution and reduction of the charge effect is only expected. However, if the rate of deceleration is too large, an aberration effect resulting from a newly occurring decelerator would delete the above expected improvement.
As can be seen from the above description, these apparatuses of the prior art do not fundamentally solve the above problems.
With this energy filter, the feature of accelerating the secondary ions and decelerating the ions immediately before the quadrupole mass spectrometer has been disclosed in "International Journal of Mass Spectrometer and Ion Physics, 43 (1982) P.31-P.39 (Elsevier Scientific Publishing Company)" and "Detection of Hydrogen in Metals by the SIMS-Method with Quadrupole Mass Filter," Vol. 16, No. 2, February 1977, pp. 335-342 (Japanese Journal of Applied Physics). In these conventional apparatus, the ion trajectory unit of the energy filter is kept at the ground potential, and the center potential of the sample and quadrupole is biased to a higher level. However, there is no difference in their practical effects.
Moreover, there is an example of prior art elementary analyzers using a magnetic sector mass spectrometer (instead of the quadrupole mass spectrometer) to cause secondary ions to pass through an ion optical system and the magnetic sector mass spectrometer under an energy condition higher than that when ions are emitted. In this case, the collection efficiency as to the secondary ion emitting angle distribution is high, but the collection efficiency as to the energy distribution which would obstruct the improvement of sensitivity is considerably lower. This results from the fact that it is necessary to make narrow the energy range of secondary ions passing through the magnetic sector mass spectrometer due to its inherent characteristics and the energy range in the actual apparatus becomes 1-2 eV.
Moreover, there is another example of prior art elementary analyzers using a time of flight mass spectrometer through which ions are caused to pass under an energy condition higher than that when the ions are emitted. In this case, collection efficiencies as to the secondary ion emitting angle distribution and the energy distribution are high. However, the excitation time for the secondary ions must be less than 10.sup.-7 seconds as a single-shot excitation owing to the inherent characteristic of the time of flight mass spectrometer, so that this elementary analyzer could not effect continuous ion detection and therefore the ultimate sensitivity becomes very low.
As above described, the elementary analyzers of the prior art have disadvantages such as low collection efficiency for the secondary ion emission angle distribution and energy distribution, influence of chromatic aberration for the space resolution, influence of fringing effect and charge effect, impossibility of continuous measurement and the like. Accordingly, the elementary analyzers of the prior art do not exhibit sufficient sensitivity and accuracy and could not effect the superior elementary analysis in space resolution.
Particularly, the low sensitivity is a prohibitive disadvantage which could not be eliminated by any means. Such prior art apparatuses could not effect the trace elementary analysis with an impurity limit concentration less than 10.sup.15 atoms/cc in monatomic layers and the local analysis of submicron or quartermicron as lower limitations to be integrated for semiconductor devices. These analyses are very important in the present semiconductor industry. The applicant has already proposed the mass spectrometer capable of analyzing an insulator and of mass-analyzing the secondary ions involving more than two ranges of the secondary ion energy constant, as disclosed in U.S. Pat. No. 4,652,753 issued on Mar. 24, 1987. This prior art mass spectrometer has an arrangement similar to that of the present invention but is based upon the novel idea that it is not necessary to pass the secondary ions through the energy filter and quadrupole type mass spectrometer while these secondary ions are being accelerated, which is in fact implemented in the present invention. Simple, the convention spectrometer spreads the analyzable energy range.
To expand the energy range of the energy filter, the resolution must be set to be high in view of the construction, or the transmission energy must be selected to be high. Moreover, the potential of the energy filter needs to be low even if the potential change caused by charging up the sample is shifted to lower levels (i.e., the polarity opposite to that of the secondary ions). The mass analysis can still be performed. To achieve one of the reliable and simple ways to lower the energy range, as previously described, the conventional mass spectrometer is arranged in the similar construction to that of the present invention. However, the conventional mass spectrometer must be operated under the different conditions.